Why is (Diet) Soda Bad for You?

I drink Diet Coke everyday, at least twice a day. If regular soda like Dr. Pepper didn't have so many calories, I'd drink it too, but I can't get past the idea of consuming that many empty, sugar-filled calories each day. So, I avoid "regular soda" like the plague.

I've been told in order to: Be healthy/lose weight you have to stop drinking soda, even diet soda. I'm interested in becoming more healthy, but can't wrap my mind around how something with zero calories can adversely affect your health/weight. So, why is diet soda so bad for you? Is it really a habit I should kick?

I guess I should mention that along with my two Diet Cokes each day I also consume a ridiculous amount of water; at least four or five water bottles each day. Does that make a difference?